This invention relates to an apparatus and method for uniformly heating synthetic polyamide fibers which are being crimped in a stuffer box crimper and, more particularly, it relates to uniformly heating crimped fibers in the stuffer box of the crimping apparatus and the resultant product. In the production of crimped fibers from drawn tow, it is known to form primary crimp between crimper feed rolls and an adjustable counter pressure means to regulate the discharge of the crimped fibers from an elongated crimping chamber. It is also known from U.S. Pat. Nos. 4,040,155 and 3,911,539 to Hughes and Erickson et al., respectively, to inject steam into the crimped fibers as they pass through the stuffer box and to provide a means for steam removal in the crimper chamber near its discharge as well as installation of pressure sensing taps within the crimper chamber for steam pressure control. As part of Hughes' and Erickson's disclosure, the presence of a broad range of pressure zones within the crimping chamber between the point of steam injection and steam removal is established. However, the uniformity of the annealed crimped product produced in this manner is somewhat less than satisfactory because of a number of factors such as poor penetration of steam into the dense folds of crimped fibers packed in the crimping chamber which results in uncontrolled or uneven heating of the crimped fiber as it passes through the stuffer box. Non-uniform heating or large pressure gradients in the stuffing box chamber results in fibers having variable dyeability and variable degrees of twist retention along their length which results in fuzzy uneven yarns that are unsuited for production of quality carpet products.
Conventional vented stuffing boxes as described by Hughes and Erickson are designed with a series of injection holes in the upper portions of the crimping chamber and generally a lesser number of holes in the lower chamber near the exit for venting. Measurement of flow from vent ports for such devices show that when steam or compressed air is injected into a location in the upper positions of the box, most of the flow short circuits down the side walls to the first vent location. Little cross flow occurs. Short circuiting of steam flow down the side walls of the chamber due to high product density or packing in the upper regions of the crimping chamber which prevent steam penetration into the bundle and causes the steam to flow along the path of least resistance, i.e. down the side walls of the stuffing box.